1. Field of the Invention
The invention relates to a method for recovery of a halogenated hydrocarbon, particularly of an inhalation anesthetic, and to a filter for this purpose.
2. Description of the Related Art
Sorption processes are frequently used for separation, purification, and drying of gases. In this connection, there are requirements for exhaust air charged with halogenated hydrocarbons (HHCs) that boil at a low temperature, as dictated by ecological requirements.
Anesthetics that evaporate easily and are frequently used in medical practice, such as enflurane, isoflurane, sevoflurane, and desflurane, are hydrocarbons or ethers substituted with fluorine and chlorine, which are usually completely released into the surroundings during or after anesthesia, and can harm both patients and medical personnel. Furthermore, they contribute to the “ozone hole” or to the “greenhouse effect.” An estimate regarding the member states of the EU showed that in 1995 alone, pollution of the atmosphere with approximately 700 metric tons of inhalation anesthetics occurred. This amount corresponds to an additional carbon dioxide burden on the environment of 0.25% [Zeitschr. Andsthesiologie u. Intensivmed. {Journal for Anesthesiology and Intensive-Care Medicine} 6 (39), 301-306, 1998]. Both in productive processing and in the recovery of HHCs from exhaust air, particularly from expiration air of patients that is charged with inhalation anesthetics, it is a concern to work towards an economically efficient configuration of the sorption filters and related methods.
Activated charcoals that have a reactive effect are already suitable for purification of process air or exhaust air (DE 37 13 346, DE 39 35 094, and DE 40 03 668). The prerequisites for high sorption capacity combined with optimal regenerability have already been explained in the references DD 239 947, DE 36 28 858, and DE 37 31 688. The recovery of HHCs can take place at a high degree of recovery, by means of desorption at high temperatures and low pressures. In the end result of the heat treatment, however, structural damage of the sorbents and the formation of decompositions products of the HHCs, which products contain halogens, take place.
In DE 37 13 346 and DE 195 49 271, DE 42 33 577, removal of HHCs by means of zeolites is described. Zeolites have a high thermal stability for the sorption of inhalation anesthetics, and a low catalytic activity with regard to any formation of toxic products. In recent times, zeolites that are low in aluminum or de-aluminized have been used as sorption agents (DE 195 32 500).
It is known that after separation and recovery of inhalation anesthetics on activated charcoal filters or zeolite filters, other accompanying gases were simply combusted (DE 42 08 521). In this way, active substances that can be recovered are irreversibly withdrawn from the filters. In contrast, HHCs are permanently absorbed in activated charcoals having a broad pore spectrum, in the narrow pores. In the recovery of inhalation anesthetics (DE 43 08 940 and DE 195 49 271), high desorption temperatures lead to by-products that have a medically questionable effect.
Hydrophobic zeolite molecular sieves having a narrow pore distribution are used for recovery of HHCs (EP 0 284 227). Desorption takes place below 150° C. The inhalation anesthetics are condensed out and recovered. In this connection, decomposition products cannot yet be excluded.
De-aluminized zeolites adapted to the methods have already been advantageously used as sorbents (DE 197 49 963). The sorbed HHCs are desorbed by means of heating, condensed, and recovered. Because of high vapor pressures of the anesthetics, condensation must take place in a range of 2° C. to 8° C. The desorption of isoflurane takes place under vacuum (approximately 10 mbar), and with simultaneous heating to about 100° C. to 160° C. The maximal desorption temperature therefore lies about 60° C. lower than for activated charcoal. Desflurane is desorbed between 90° C. and 130° C.
Regeneration that is gentle on the cartridge, with a steam carrier, is described in DE 101 18 768. Modified and/or de-aluminized zeolites having low water absorption below ma-2% bring about a lowering in the desorption temperature that is gentle on the sorbent and on the sorbate. It is advantageous if a saturated steam temperature under normal pressure of about 100° C. is adjusted. The condensation of the gases leads to the formation of a liquid mixture that is pre-separated in layer-like manner. The specifically lighter water layer is recirculated back into the evaporation process, while the heavier layer of the inhalation anesthetics is subsequently purified. However, possible decomposition products accumulate in the water layer.
Conventional filter arrangements have different characteristics with regard to adsorption and desorption of inhalation anesthetics, in terms of their parameters, and these are significantly dependent on the conditions for flow and temperature. In order to achieve uniformity of the process management without any time delay (hysteresis), a different energy feed is provided for a filter cartridge, for example, whereby the adsorbed anesthetics can be released again from the interior of the cartridge, in targeted manner (EP 0 611 174, EP 1 222 940). Sorbents that have different effects are not yet used in combination. Also, specially shaped configurations of filter inserts for respiration gases are usual, in order to use up the contents uniformly, even at higher flow speeds (DE 36 12 924) and to prevent local break-throughs through the filter layer.
In a proposal by the applicant, “Filterpatrone zur Rückgewinnung niedrigsiedender halogenierter Kohlenwasserstoffe {Filter cartridge for recovery of halogenated hydrocarbons that boil at low temperatures},” the gas pass-through in the upper wall region of a single filter insert is configured in such a manner that a plug flow for the gases occurs in this region, thereby achieving uniformity of the break-through curves for inhalation anesthetics. The anesthesia gases used therefore have “good” break-through curves at the upper edge of the filter insert having the hydrophobic zeolites, in other words with a very steep characteristic of the transition, which is clearly determined in terms of both location and time, in that a sharp boundary forms between the charged and not yet charged parts of the zeolite fill.
The filter system, which is complex in its effect, is difficult to understand clearly and permits a possible determination of best values for its time sequence only on the basis of long-term and empirical work experience that has been gained. In particular, placement in multiple sorbent beds is problematical, where filters that can be handled in medical technology, in particular, are supposed to demonstrate a steep characteristic of the break-through curve, with times that can be precisely determined.
Multi-Stage Filter Arrangements
There has been no lack of attempts to increase the degree of charging of sorption beds during targeted purification of gas streams. In DE 43 19 327, an untreated gas stream is passed through two sorption beds, one after the other. After the process has been completed, the first sorbent bed is regenerated and the flow direction is reversed, so that the flow passes through the second bed first. However, in this connection, the used sorbent is replaced with freshly regenerated sorbent, in complicated manner.
According to DE 198 26 684, a gas mixture is brought into contact with a sorbent at higher pressure, whereby a component of the mixture sorbs preferentially in a first work stage, and is desorbed in a second work stage, under reduced pressure. The two regions are separated from one another in such a manner that only the sorbed component passes over into the second region. Rectification of gas components that boil at low temperature is combined with alternating-pressure adsorption. The mechanical device for this has a complicated structure and does not meet the requirements for simple filter arrangements.
It is also state of the art that an improvement in sorptive separation is made possible by means of physical differences, for example in the pore sizes and by means of changes in the sorbent beds themselves. In the case of gas components that are not the same, like the ones that form the mixture components of laughing gas and anesthetic vapors in DE 197 06 806, their selective separation can take place by means of different types of molecular sieves. These can be used mixed with one another, or can be placed in two sorbent beds that are different from one another. In this connection, they have different pore sizes, which also could lead to sequences of the sorption processes that are different in terms of time. Two molecular sieve regions are provided, each having different ranges of pore sizes, of 0.8-1 nm and 0.3-0.5 nm, and flow takes place through them one after the other, in terms of space. While good adsorption properties are claimed for both gases, these are made possible on the basis of purely steric and thus static influences in the adjustment of the sorption equilibria. Recover by means of a carrier gas or carrier vapor is not yet taken into consideration. In particular, it is not taken into consideration that when sorption capacities that are higher than those present with zeolite molecular sieves, such as in the case of carbon molecular sieves, special dynamic influences occur, which additionally improve the sorption properties as well as the recovery of the anesthetics by means of regeneration using steam.
Process Management of HHC Filters:
In previous filter arrangements, the process management is essentially controlled, to some extent, by means of conditions that can be established macroscopically, and according to geometry parameters and operating parameters. In contrast, the separation precision of substance separation by means of sorption is necessarily determined by microscopic parameters. Differences in the molecule size in the spatial lattice structure of the sorbents bring about static sieve effects and blockages when passing through the lattice. In contrast to this, the time progression of the separation processes in the related filters is established by means of kinetics that have a complicated and dynamic effect, with marked non-idealities.
Improvements in sorptive substance separation can be provided by means of changes in the process management. In a monograph, “Adsorptionsverfahren zur Wasserreinigung {Adsorption Methods for Water Purification}” by Sontheimer, Frick, Fettig, Horner, Hubele, and Zimmer, DVWG-Forschungsstelle {test laboratory} at the Engler-Bunte Institut {institute} of the Technical University of Karlsruhe, Karlsruhe 1985, methods of procedure that have at least two stages, with flow in the same direction, for sorption beds disposed one behind the other, are proposed; their calculations lead to the best possible values of the end concentration after the first or additional sorption bed, and allow an advantageous distribution of the masses of similar types of sorbents on multiple sorption beds at a predetermined total mass. These recognitions are based on adjusted sorption equilibria and can be transferred to filter arrangements for HHCs. In particular, it has become known that carbon molecular sieves that are suitable for the sorption of inhalation anesthetics often demonstrate a greater charge than zeolites. However, in contrast to zeolites, carbon molecular sieves tend to have a disadvantageous break-through behavior, with insufficient utilization of the capacity of the filters. It is disadvantageous that the desorption curves of activated charcoals run flatter, while it is advantageous that their sorption capacity is greater.
Zeolites already possess high charge values for halogenated hydrocarbons. However, activated charcoals demonstrate a higher substance throughput at lower desorption temperatures, and thus can purify the exhaust air better.
In the determination of break-through times in filter arrangements from break-through curves, it turns out that in the event of desorption of a filter with a zeolite, shorter times to break-through are set, with a steep progression of the curves. In contrast, longer break-through times are observed when using carbon molecular sieves, with a flat progression of the break-through curves. The individual static and dynamic influence variables are difficult to separate from one another in the case of filters for HHCs and inhalation anesthetics, and therefore can hardly be investigated independent of one another. For this reason, it is difficult to make use of the individual advantages of the use of sorbents that have different effects, without optimization. For this reason, it is an urgent concern to establish the set-up of at least two sorbent beds of a filter arrangement that work together as practically as possible.
It has not yet been found that a combination of hydrophobic zeolites with activated charcoals is practical for determining the best values of proportions in at least two sorption beds of filters for inhalation anesthetics.